Documentation: prctl/seccomp_filter
Documents how system call filtering using Berkeley Packet
Filter programs works and how it may be used.
Includes an example for x86 and a semi-generic
example using a macro-based code generator.
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
v18: - added acked by
- update no new privs numbers
v17: - remove @compat note and add Pitfalls section for arch checking
(keescook@chromium.org)
v16: -
v15: -
v14: - rebase/nochanges
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - comment on the ptrace_event use
- update arch support comment
- note the behavior of SECCOMP_RET_DATA when there are multiple filters
(keescook@chromium.org)
- lots of samples/ clean up incl 64-bit bpf-direct support
(markus@chromium.org)
- rebase to linux-next
v11: - overhaul return value language, updates (keescook@chromium.org)
- comment on do_exit(SIGSYS)
v10: - update for SIGSYS
- update for new seccomp_data layout
- update for ptrace option use
v9: - updated bpf-direct.c for SIGILL
v8: - add PR_SET_NO_NEW_PRIVS to the samples.
v7: - updated for all the new stuff in v7: TRAP, TRACE
- only talk about PR_SET_SECCOMP now
- fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
- adds dropper.c: a simple system call disabler
v6: - tweak the language to note the requirement of
PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
v5: - update sample to use system call arguments
- adds a "fancy" example using a macro-based generator
- cleaned up bpf in the sample
- update docs to mention arguments
- fix prctl value (eparis@redhat.com)
- language cleanup (rdunlap@xenotime.net)
v4: - update for no_new_privs use
- minor tweaks
v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
- document use of tentative always-unprivileged
- guard sample compilation for i386 and x86_64
v2: - move code to samples (corbet@lwn.net)
Signed-off-by: James Morris <james.l.morris@oracle.com>
diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c
new file mode 100644
index 0000000..579cfe3
--- /dev/null
+++ b/samples/seccomp/bpf-helper.c
@@ -0,0 +1,89 @@
+/*
+ * Seccomp BPF helper functions
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_ATTACH_SECCOMP_FILTER).
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "bpf-helper.h"
+
+int bpf_resolve_jumps(struct bpf_labels *labels,
+ struct sock_filter *filter, size_t count)
+{
+ struct sock_filter *begin = filter;
+ __u8 insn = count - 1;
+
+ if (count < 1)
+ return -1;
+ /*
+ * Walk it once, backwards, to build the label table and do fixups.
+ * Since backward jumps are disallowed by BPF, this is easy.
+ */
+ filter += insn;
+ for (; filter >= begin; --insn, --filter) {
+ if (filter->code != (BPF_JMP+BPF_JA))
+ continue;
+ switch ((filter->jt<<8)|filter->jf) {
+ case (JUMP_JT<<8)|JUMP_JF:
+ if (labels->labels[filter->k].location == 0xffffffff) {
+ fprintf(stderr, "Unresolved label: '%s'\n",
+ labels->labels[filter->k].label);
+ return 1;
+ }
+ filter->k = labels->labels[filter->k].location -
+ (insn + 1);
+ filter->jt = 0;
+ filter->jf = 0;
+ continue;
+ case (LABEL_JT<<8)|LABEL_JF:
+ if (labels->labels[filter->k].location != 0xffffffff) {
+ fprintf(stderr, "Duplicate label use: '%s'\n",
+ labels->labels[filter->k].label);
+ return 1;
+ }
+ labels->labels[filter->k].location = insn;
+ filter->k = 0; /* fall through */
+ filter->jt = 0;
+ filter->jf = 0;
+ continue;
+ }
+ }
+ return 0;
+}
+
+/* Simple lookup table for labels. */
+__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label)
+{
+ struct __bpf_label *begin = labels->labels, *end;
+ int id;
+ if (labels->count == 0) {
+ begin->label = label;
+ begin->location = 0xffffffff;
+ labels->count++;
+ return 0;
+ }
+ end = begin + labels->count;
+ for (id = 0; begin < end; ++begin, ++id) {
+ if (!strcmp(label, begin->label))
+ return id;
+ }
+ begin->label = label;
+ begin->location = 0xffffffff;
+ labels->count++;
+ return id;
+}
+
+void seccomp_bpf_print(struct sock_filter *filter, size_t count)
+{
+ struct sock_filter *end = filter + count;
+ for ( ; filter < end; ++filter)
+ printf("{ code=%u,jt=%u,jf=%u,k=%u },\n",
+ filter->code, filter->jt, filter->jf, filter->k);
+}